Preheat oven to 350°.
Grease springform pan with butter.
Mix ricotta, cream cheese and sugar.
Add eggs, one at a time. Beat well.
Add sour cream with flour and vanilla extract. Beat until smooth, taking care not to overmix.
Pour into prepared springform pan. Place pan into waterbath and into oven.
Bake for 90 minutes. Turn oven off, but leave cake in oven with the door open for another 60 minutes.
Remove cake and cool completely on wire rack.
Chill cake for several hours. Before serving, remove outer ring of springform pan and top as desired.